Roebel bars in the active parts of the stator winding of a turbogenerator are well-known in the art (see the documents cited below).
Directly gas (hydrogen) cooled Roebel bars of turbogenerator stator windings consist of two or four stacks of individual strands. Between them non-transposed cooling tubes are located. In single Roebel bars the cooling tube stack is located between the two stacks of strands. This is as well the case of a double Roebel bar with a cooling tube stack in the middle of each single bar (in the total bar there are two cooling tube stacks, see document DE 100 59 385 A1 FIG. 1 a). These arrangements need a transposition of the strands crossing the cooling tube stack. At bar ends the individual strands or bundles of them are connected to the next bar by “transposing” the strands additionally.
In the case of having a double Roebel bar consisting of two single Roebel bars and a single cooling tube stack between them the strands of the single Roebel bars are transposed internally without crossing the cooling tube stack. FIG. 1 shows the cross section of a stator winding 10 with such double Roebel bars 14a and 14b. Each double Roebel bar 14a,b comprises two single Roebel bars 17 and 18, with a stack of cooling tubes 16 disposed between them. The double Roebel bars 14a,b are each surrounded by an insulation 15 and arranged within a slot 12 of the stator body 11. A wedge 13 keeps the bars in the slot 12.
FIG. 2 shows the top view on the bar 14b. Each single Roebel bar 17, 18 comprises (in this simplified example) two parallel stacks of individual strands 17a-c, 17d-f and 18a-c, 18d-f. Within each of the single Roebel bars 17, 18 the respective strands 17a-f and 18a-f are internally transposed with no crossing of the stack of cooling tubes 16. The transpositions are done in series of three 180°-transpositions, so that a total (added) Roebelisation angle of 540° is achieved.
At bar ends the individual strands (or bundles of them) are again connected to the next bar by “transposing” the strands additionally.
At bar ends it is possible to put one massive lug per single bar. In total there are two lugs per bar end, see FIG. 3a, where normal massive double lugs 19a and 19b are provided to connect the single Roebel bars of double Roebel bars 14a and 14b. 
In the middle of a phase group a crossing lug has to be foreseen to compensate for the stray field voltages which are collected along the two parallel paths which are formed by the single bars of all the double Roebel bars, see FIG. 3b, where two crossing double lugs 20a and 20b are provided to connect the single Roebel bars of double Roebel bars 14a and 14b crosswise.
Document DE 100 59 385 A1 discloses a device, which has sub-conductors divided into 4 adjacent stacks and 2 rows of cooling lines, each between both inner sub-conductor stacks and one of the outer stacks. Both inner stacks form a core rod in which only the sub-conductors of the two inner stacks are twisted together. The two outer stacks form a hollow rod in which only the sub-conductors of the two outer stacks are twisted together and the cooling lines pass through without pinch points. The device has a number of sub-conductors divided into four adjacent stacks and two rows of cooling lines, each between both inner sub-conductor stacks and one of the outer stacks. Both inner stacks form a core rod in which only the sub-conductors of the two inner stacks are twisted together. The two outer stacks form a hollow rod in which only the sub-conductors of the two outer stacks are twisted together and the cooling lines pass through without pinch points. Independent claims are also included for the following: a method of manufacturing a dual twisted rod.
Document DE 197 54 943 A1 discloses a winding for the stator of an electrical machine comprises an electrical conductor which forms the part of the winding stipulated for insertion into a slot in the stator with its end sections. The conductor is formed out of a number of subconductors located in four adjacent stacks. The stacks have the same number of subconductors which are twisted together over the length of the conductor. Each two adjacent subconductors are twisted. The twisted subconductors are bent together at one or more places over the length of the conductor so that the previously inside-lying subconductor behind the bend point lies outside and the previous outside-lying conductor behind the bend point lies inside. The subconductors are also, or instead, bent in crosswise fashion at one or more places over the length of the conductor so that the previously inside-lying subconductor behind the bend point also lies inside and the previously outside-lying subconductor behind the bend point also lies outside.
Document EP 2 262 079 A1 discloses a stator bar, which comprises four stacks of strands defining an active part wherein the strands are transposed by successive crossovers from one stack position to another, and two end windings extending from the two ends of the active part. The strands of the active part are transposed by 360° or 540° such that all strands occupy all positions in the bar for substantially equal distances. In addition, the end windings are transposed by successive crossovers from one stack position to another by an angle between 60-180°.
Document U.S. Pat. No. 3,647,932 discloses a transposed stranded conductor for dynamoelectric machines having four stacks of strands transposed in the slot portion of the conductor in such a manner that each stack moves to different transverse positions in successive longitudinal sections of the bar such that the stacks are reversed in transverse position at opposite ends of the bar. Preferably, the strands are also transposed in a manner to cause inversion of the relative strand positions at opposite ends of the slot so that the conductor is fully transposed with inversion of strand position both transversely of the slot and depthwise of the slot.
While certain Roebelisations are known for indirectly cooled Roebel bars and directly water cooled Roebel bars (where the cooling conductors are transposed as well as the massive strands), see the already mentioned documents U.S. Pat. No. 3,647,932 and DE 197 54 943 A1, above, no such Roebelisations are known for directly gas cooled Roebel bars (with non-transposed cooling tube stacks put in the bars) comprising double Roebel bars.
The known solution for the Roebel bar design with massive lugs including crossing lugs or similar, as shown in FIG. 3, requires a substantial amount of space, especially when crossing lugs are involved.
It would therefore be advantageous, especially for retrofit applications, to have a Roebelisation design, which requires less space.